The invention relates to a picture display device comprising:
a display tube (CRT) and deflection means (L) for deflecting an electron beam in the display tube (CRT) at a deflection frequency, and PA1 a focusing circuit (FC) comprising an input for receiving an input signal (flb) synchronized with the deflection frequency and an output for supplying a focusing signal (Vf) for the display tube (CRT), the focusing signal (Vf) having a repetition frequency which is equal to the deflection frequency, the focusing circuit (FC) comprising a multiplier (3; IC3) receiving a signal being related to the input signal (flb).
The invention further relates to a focusing circuit as described in the introductory part of claim 7.
The invention also relates to a method of generating a display tube focusing signal.
From U.S. Pat. No. 4,216,392 is known to generate a focusing waveform suitable for driving a focusing electrode of a cathode ray tube. The known circuit generates a parabolic focusing waveform by multiplying a sawtooth waveform synchronized with the deflection frequency by itself.
It is difficult to prevent a shifting of DC-levels if the sawtooth waveforms are multiplied by itself. If higher order waveforms are desired, these DC-levels produce undesired low-order components in the output signal which is generated by multiplication of the sawtooth waveform by itself.